Method and system for utilizing enthalpy contained in exhaust gases of low-temperature fuel cells

ABSTRACT

A method for utilizing the heat in the exhaust gases of a low-temperature fuel cell module is disclosed, whereby the exhaust gases from the fuel cell are introduced into a condenser for energy acquisition. A system for the implementation of the inventive method is also disclosed.

FIELD OF THE INVENTION

The invention is directed to a low-temperature fuel cell system withforce-heat coupling, as well as to a method for utilizing the waste heatof a low-temperature fuel cell module.

BACKGROUND OF THE INVENTION

German Letters Patent 42 34 151 discloses that the heat from the exhaustgases of high temperature fuel cells exploitable via heat exchangers andvia normal flow heaters. German Letters Patent 40 32 993 discloses thatthe anode exhaust gas of a fuel module, which can, among other things,also be a low-temperature fuel cell module, can be utilized forcombustion. It is provided in the embodiment with high-temperature fuelcells contained in this Prior Art that the hot exhaust gases from thehigh-temperature fuel cells are utilized for pre-heating the as yetunheated fuels that are rich in H₂. Up to now, however, no method and nosystem for the implementation of a method has been disclosed wherein theenthalpy contained in the exhaust gases of low-temperature fuel cells isexploited for increasing the overall efficiency of an energy supplysystem.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to offer a method aswell as a system for the implementation of a method wherein the enthalpyor condensation energy contained in the exhaust gases of alow-temperature fuel cell module contributes to increasing the overallefficiency of the energy conversion system.

The invention is based on the perception that the condensation energycontained in the exhaust gases of a low temperature fuel cell can beconsiderable, particularly when it is operated at low pressure, and canbe utilized in the overall system of an energy supply system. Inparticular, there is the underlying perception that the combination oftwo heat exchangers following a low-temperature fuel cell module--first,a heat exchanger that makes the heat stored in the coolant usable and,second, a heat exchanger that makes the enthalpy contained in theexhaust gases usable--increases the thermal output and, thus, the degreeof thermal utilization of the low-temperature fuel cell module such thatan overall degree of utilization of the hydrogen fuel of greater than100% can be achieved with reference to H_(u) of H₂ (net calorific value"H_(u) ").

The subject matter of the invention is a method for the utilization ofthe waste heat of a low-temperature fuel cell module, whereby exhaustgas from the low-temperature fuel cell module that is heated andenriched with vaporous reaction product is introduced such into a heatexchanger that at least the vaporous reaction product contained in theexhaust gas condenses out and the energy being thereby released isrendered usable, as well as a low-temperature fuel cell system withforce-heat coupling wherein the exhaust gas is connected to a heatexchanger that comprises a condenser.

The heat exchanger which makes the enthalpy of the exhaust gas usablecomprises a condenser. The remaining configuration of the heatexchanger, however, is not intended to limit the scope of the inventionin any way whatsoever because other energy converters that convert theenthalpy of the exhaust gas into other usable energy can also beinventively utilized here.

For example, all types of fuel cells whose operating temperature liesbelow that of the MCFC (molten carbonate fuel cell: operatingtemperature approximately 600° C.) can be utilized as fuel cells of thelow-temperature fuel cell system. Let the PAFC (phosphoric acid fuelcell: operating temperature approximately 150°C.-250°C.), DMFC (directmethanol fuel cell: operating temperature approximately 80° C.-150° C.),PEM (membrane fuel cell: operating temperature approximately 50° C.-80°C.), as well as the AFC (alkaline fuel cell: operating temperature 60°C.-90° C.) be thereby cited by way of example.

The use of the polymer membrane (PEM) fuel cell is especially preferred.What is understood by polymer membrane fuel cell is the PEM fuel cell,whereby PEM stands for polymer electrolyte membrane or proton exchangemembrane. The designations SPE (solid polymer electrolyte), SPFC (solidpolymer fuel cell), PEFC (polymer electrolyte fuel cell) and IEM (ionexchange membrane) fuel cell are also frequently additionallyencountered. These are thereby a matter of fuel cells withmacro-molecular membrane electrolytes.

A system is also especially advantageous wherein the energy stored inthe used and heated coolant is also rendered usable via a heat exchangerin addition to the utilization of the exhaust gas enthalpy. What isespecially advantageous about this is that both the heat exchanger forthe coolant as well as the heat exchanger for the exhaust gas can beintegrated in a single unit in space-saving fashion. It is especiallyadvantageous given this embodiment when the coolant can be conducted incirculation , i.e. when it is returned to the fuel cell module aftercooling and regeneration.

A system with a PEM fuel cell module is also advantageous wherein, in acondenser, the enthalpy from the exhaust gases of the fuel cell module

is converted into usable energy (for example thermal energy), on the onehand, and

on the other hand, the condensed product of the fuel cell reaction, i.e.water, is returned into the PEM fuel cell module for moistening.

Given this embodiment, the product water arising in liquid form canthereby be supplied via extra lines into the feeder for the reactants,usually the reaction gases. It then serves for moistening the reactiongases or for moistening the membrane as well. Further, the waterobtained from the condenser can also be utilized in some other way (forexample water for domestic use).

Given the inventive method, either all of the exhaust gas, i.e. anodeand cathode exhaust gas together, can be introduced into the heatexchanger or only one exhaust gas, i.e. either anode or cathode exhaustgas by itself.

The inventive method is preferably operated at an operating temperatureof the fuel cell module between 30° C. and 150° C. The temperatureranges between 50° C. and 100° C. are especially preferred andtemperature ranges between 45° C. and 80° C. or an operating temperatureof approximately 70° C. are particularly preferred.

It is also advantageous when the residual heat of the exhaust gas in theinventive method is further utilized for heating purposes after it hasleft the condenser. In particular, it is possible to directly supply atleast a part of the exhaust gas emerging from the heat exchanger intothe ventilation of the building to be heated. However, the exhaust gasemerging from the heat exchanger or condenser can likewise be introducedinto a further heat exchanger via lines specifically provided therefor.It can thereby serve for heating fresh water.

"Fuel cell module" is the term for a unit that comprises the actualelectrochemical converter, the actuating elements and sensors pertainingthereto and the appertaining humidifier. What is referred to as "fuelcell system" is a system that comprises the module and the appertainingsystem periphery. The selection of the verb "comprising" is therebyintended to express that the two terms (. . . module and . . . system)do not limit the units to these features but can also have more andother component parts.

As stated, all low-temperature fuel cells come into consideration asfuel cells in the inventive method and the system therefor. The PEM fuelcell is preferably involved. The reactants converted in the fuel cellsare not subject to any limitation whatsoever within the scope of theinvention; all gases and liquids that can act as oxidant or fuel in fuelcells are thereby involved. Let air, oxygen and arbitrary mixtures ofthese components as well as hydrogen, methanol, synthesized and/orreformer gas as well as natural gas be cited by way of example.

The nature of the coolant is in turn based on the nature of the fuelcell employed, whereby deionized water or some other, electricallynon-conductive medium, for example ethylene glycol, air or any gas, canbe cited by way of example for the preferably utilized PEM fuel cellblocks. A heat-pipe cooling is also possible, whereby the heat exchangeris the condenser for the heat-pipe medium. What is meant by heat-pipecooling is that the part to be cooled is connected by athree-dimensional channel ( "pipe") to a cooler part, whereby the liquidcontained in the pipe evaporates at the part to be cooled and in turncondenses out at the cooler, second part.

What is referred to as "anode exhaust gas" is the exhaust gas thatleaves the anode chamber of the fuel cell, i.e. the exhaust gas of thefuel of the fuel cell. Likewise, what is referred to as "cathode exhaustgas" is the exhaust gas that leaves the cathode chamber of the fuelcell, i.e. the exhaust gas of the oxidant.

The method is preferably utilized in stationary energy supply systems;however, an employment in mobile energy supply systems as well is notprecluded. The stationary energy supply systems are thereby not only amatter of industrial-scale systems; rather, individual houses orresidential complexes can also be equipped with an energy supply systemthat is operated with the inventive method. The energy supply systemcomprises a power and heat supply, whereby the heat can be used for thepurpose of room heating and/or for preparing warm or hot water. Theelectrical energy can be used in an E-store, for example battery orflywheel, the thermal energy in a heat store. In general, the inventiveprinciple can be realized in all possible dimensions, whereby theemployment in stationary heating systems such as, for example, inresidential complexes was in the foreground in the development of theinvention. Accordingly, the terms "fuel cell module" and "system" canalso not be dimensionally fixed because, of course, they can drasticallyvary dependent on the field of employment.

The inventive heat exchanger for the exhaust gas, which can also be anenergy converter, uses the energy of the exhaust gas that, first,becomes free when the gas is brought to a lower temperature and that,second, is freed as condensation energy in the liquidization of thevapor. The energy of the attracting molecular forces, i.e. thecondensation energy, thereby constitutes by far the greater part. Thisenergy is equal to the specific evaporation energy of the liquid. It hasan especially high value given water, namely 4.06×10⁷ J/mol or 2.25×10⁶J/kg. Inventively, this energy is utilized via a unit following upon thefuel cell.

Given the preferably utilized PEM fuel cell module, the product water isformed at the cathode side on which the oxidant flows. Although thevapor-saturated cathode exhaust gas is correspondingly preferablyintroduced into the heat exchanger, product water can also always befound at the anode side as a consequence of diffusion because of thegreat differences in concentration of water within each and every fuelcell, and the anode exhaust gas can also be productively introduced intoa heat exchanger.

Further, the product water that has arisen in the heat exchanger orcondenser cannot only be re-employed and, in particular, utilized formoistening the reactants or the membrane, the enthalpy remaining in the"second exhaust gas" after condensation of the product can also besimultaneously released. Differing from the exhaust gas that exits thefuel cell, the exhaust gas that leaves the heat exchanger, energyconverter or condenser and that is colder is referred to here as "secondexhaust gas". Nonetheless, this second exhaust gas also still hasunexploited energy and enthalpy that can be productively used. Inparticular, the second exhaust gas can be directly supplied into theroom air for heating. In addition, it can also be partially suppliedinto the room air and partially subjected to a further energy converteror heat exchanger.

"Condenser" is used here in the usual sense as generally standardsynonym for a condensation heat exchanger. What is meant by "reformer"is an H₂ generator from hydrocarbon compounds (for example natural gasor methanol).

In an embodiment, the method of the present invention comprises a methodfor utilizing heat generated by a low temperature fuel cell module whichproduces exhaust gas that includes air and reaction product vapor. Themethod of the present invention includes the steps of passing theexhaust gas through a heat exchanger, condensing at least part of thereaction product vapor in the heat exchanger as the exhaust gas ispassed therethrough thereby generating heat of condensation, passing afluid medium through the heat exchanger as the reaction product vapor iscondensed therein and heating the fluid medium with the heat ofcondensation generated by the condensing of the reaction product vaporto generated a heated fluid medium.

In an embodiment, the fuel cell module further produces a stream ofheated coolant and the method of the present invention further comprisesthe step of passing the heated coolant through the heat exchanger andtransferring heat from the heated coolant to the fluid medium.

In an embodiment, the heated fluid medium is used to provide heat for abuilding or a home.

In an embodiment, the air component or secondary exhaust gas componentof the exhaust gas contains heat as it departs the heat exchanger and,therefore, in such an embodiment, the heat contained in the aircomponent of the exhaust gas or the "secondary exhaust gas" issues forheating purposes. In such an embodiment, heat contained in the secondaryexhaust gas can be used to heat a second fluid medium or to directlyheat a room or building in the event the secondary exhaust gas is air.

In an embodiment, the present invention provides a polymer membrane fuelcell with force-heat couplings that generates an exhaust gas thatincludes reaction product vapor. The system further comprises a heatexchanger through which the exhaust gas passes and a condenser forcondensing at least some of the reaction product vapor and generatingheat of condensation. The heat exchanger transfers the heat ofcondensation to a fluid medium.

Other objects and advantages of the present invention will becomeapparent from reading the following detailed description and appendedclaims, and upon reference to the accompanying drawings.

The above definitions are valid for the claims, the specification andthe descriptions of the Figures. The invention shall now be explainedbelow on the basis of an example that shows domestic energy supply withPEM fuel cells upon utilization of the inventive method:

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIGS. 1 and 2 illustrate schematically, the system technology ofstationary applications of the inventive method.

It should be understood that the drawings are not necessarily to scaleand that the embodiments are sometimes illustrated by graphic symbols,phantom lines, diagrammatic representations and fragmentary views. Incertain instances, details which are not necessary for an understandingof the present invention or which render other details difficult toperceive may have been omitted. It should be understood, of course, thatthe invention is not necessarily limited to the particular embodimentsillustrated herein.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

The block circuit diagram of the system can be seen in FIG. 1. The fuelcell module 1 with the two feeds 2 and 3 is in the middle of theillustration. The oxidant (air) is supplied to the fuel cell module 1via the feed 2 and the fuel (H₂) comes via the feed 3. In addition toair, the oxidant can thereby also be oxygen or arbitrary mixtures of thetwo and the fuel can be hydrogen, methanol, synthetic and/or reformergas as well as natural gas. In the case of reformer gas or methanol, thefeed 3 can also be preceded by a reformer 4. The exhaust gas leaves thefuel cell module via the outlet 5 and reaches the condenser 6. Theexhaust gas is cooled in the condenser 6 and the product water iscondensed out. The conduit 7 conveys the water heated via the condenser,first, to the warm water store 9 via the connection 8 and, second, tothe heat exchanger 11 via the conduit 10, said heat exchanger 11 beingdirectly connected to the household. The "second exhaust air" and thecondensed product water leave the condenser 6 at the other side of thecondenser 6, at the outlet 13. To the left of the fuel cell module 1,the generated electricity is conducted to the current transformer 12that transforms the direct current generated by the fuel cell moduleinto alternating current. The alternating current can then be supplied,on the one hand, into the household and, on the other hand, into thepublic network.

FIG. 2 shows a further diagram of the household energy supply with PEMfuel cells upon heat output given hydrogen/air operation. The fuel cellmodule 21, which is supplied with hydrogen and air via two feeds 22 and23, is at the far left in the Figure. The cooling circulation, in whichthe coolant is conducted in circulation between the heat exchanger ofthe unit 26 and the fuel cell module 21, is located at the rightfollowing the fuel cell module. The coolant flows into the fuel cellmodule with a temperature of, for example, 50° C. and leaves it with atemperature of, for example, 70° C. Via a pump 27, the approximately 70°C. hot outflow proceeds into the unit 26 and into the heat exchangertherein, the latter being connected to a heating circulation and theoutflow giving off thermal energy therein while being cooled toapproximately 50° C. The coolant is in turn regenerated with thistemperature and can be introduced into the fuel cell module.

The exhaust air from the fuel cell module 21 saturated or at leastenriched with water vapor proceeds via the conduit 25 to the unit 26that also has a condenser integrated in addition to the heat exchanger.The water vapor contained in the exhaust air is condensed in thecondenser of the unit 26 to water that leaves the unit 26 as usefulwater via the outlet 28. The condensation heat being released and theheat of the exhaust air as well as the heat of the coolant are used tosupply the heat circulation connected to the heat exchanger andcondenser unit 26 with thermal energy. The heating agent conducted incirculation in the heating circulation enters, for example, into theunit 26 with a temperature of, for example, 45° C. and leaves the unitwith, for example, 65° C. having been heated by approximately 20° C. Itthen proceeds via a pump 30 to a distributor 31 via which it ispartially conducted into the heating element 32 and partly into the warmwater store 33. The warm water store is supplied with tap water at 10°C. and can heat the tap water in this system to approximately 60° C.Current, which is again conducted through a d.c./a.c. transformer, isacquired for the electricity supply of the house at the other side ofthe fuel cell module 21 (line 32).

The inventive system can make both electricity as well as heat forheating purposes and for preparing warm water available. The heatingcirculation can also be omitted; the heat is then used only forpreparing warm water.

The temperatures indicated in FIG. 2 are preferred operatingtemperatures; the system, however, works up to an operating temperatureof the fuel cell module of approximately 30° C. through 40° C. whileincreasing the thermal efficiency of the overall energy supply system.The water acquired from the heat exchanger and condenser unit 26 can beintroduced into a service water system and can also be partly orentirely utilized for moistening the membrane of the fuel cell module.The exhaust air emerging from the unit 26 via the conduit 29 has aresidual caloric content that can in turn be used either directly forheating rooms or via a further heat exchanger, the latter particularlywhen the system uses the heat mainly for preparing warm water. Thestructural design of the unit 26 can comprise all possible forms of thecombination of heat exchanger and condenser, whereby the efficiency ofthe overall system becomes all the higher the lower the energy contentof the exhaust air emerging from the unit 6.

What is claimed is:
 1. A method for utilizing heat generated by a fuelcell module which produces exhaust gas comprising air and reactionproduct vapor and a stream of heated coolant, the method comprising thesteps of:passing the exhaust gas through a condensing section of a heatexchanger; condensing at least part of the reaction product vapor in thecondensing section of the heat exchanger as the exhaust gas passestherethrough thereby generating heat of condensation; passing a fluidmedium through a heat exchange section of the heat exchanger as thereaction product vapor is condensed therein; heating the fluid mediumwith the heat of condensation generated by the condensing of thereaction product vapor to generate a heated fluid medium; passing theheated coolant through the heat exchange section of the heat exchanger;and transferring heat from the heated coolant to the fluid medium. 2.The method of claim 1 wherein the condensing section of the heatexchanger and the heat exchange section of the heat exchanger arecontained within a common housing.
 3. The method of claim 1 wherein theheated fluid medium is used to provide a heat supply for a building. 4.The method of claim 1 wherein the fuel cell is a polymer membrane fuelcell.
 5. A method for utilizing heat generated by a low-temperature fuelcell module which produces exhaust gas comprising reaction product vaporand secondary exhaust gas and a stream of heated coolant, the methodcomprising the steps of:passing the exhaust gas through a condensingsection of a heat exchanger; condensing at least part of the reactionproduct vapor in the condensing section of the heat exchanger as theexhaust gas passes therethrough thereby generating heat of condensation;passing a first fluid medium through a heat exchange section of the heatexchanger as the reaction product vapor is condensed therein; heatingthe first fluid medium with the heat of condensation generated by thecondensing of the reaction product vapor; removing the condensedreaction product from the condensing section of the heat exchanger;removing the secondary exhaust gas from the condensing section of theheat exchanger; heating a second fluid medium with the secondary exhaustgas, and passing the heated coolant through the heat exchange section ofthe heat exchanger; and transferring heat from the heated coolant to thefirst fluid medium.
 6. The method of claim 5 wherein the condensingsection of the heat exchanger and the heat exchange section of the heatexchanger are contained within a common housing.
 7. The method of claim5 wherein the first fluid medium is used to provide a heat supply for abuilding after it passes though the heat exchanger.
 8. The method ofclaim 5 wherein the second fluid medium is room air.
 9. The method ofclaim 5 wherein the step of heating the second fluid medium with thesecondary exhaust gas comprises:passing the secondary exhaust gas andthe second fluid medium through a second heat exchanger.
 10. The methodof claim 5 wherein the second fluid medium is air and the step ofheating the second fluid medium with the secondary exhaust gascomprises:transferring the secondary exhaust gas to a room containingthe second fluid medium.
 11. The method of claim 5 wherein thelow-temperature fuel cell comprises a polymer membrane fuel cell.
 12. Apolymer membrane fuel cell system with force-heat coupling, the systemcomprising:a polymer membrane fuel cell that generates an exhaust gascomprising reaction product vapor, the system further comprising a heatexchanger through which the exhaust gas passes, the heat exchangercomprising a condenser for condensing at least some of the reactionproduct vapor and generating heat of condensation, the heat exchangertransferring the heat of condensation to a fluid medium and whereincoolant is passed through the polymer membrane fuel cell where thecoolant is heated, and the coolant is further passed through the heatexchanger where heat from the coolant is transferred to the fluidmedium.
 13. The polymer membrane fuel cell system of claim 12 whereinthe condenser is connected to the fuel cell and the condensed reactionproduct is transmitted from the condenser to the fuel cell.
 14. Alow-temperature fuel cell system with force-heat coupling, the systemcomprising:a fuel cell that generates an exhaust gas comprising reactionproduct vapor and a secondary exhaust gas and a stream of heatedcoolant, the system further comprising a heat exchanger through whichthe exhaust gas passes, the heat exchanger comprising a condenser forcondensing at least some of the reaction product vapor and generatingheat of condensation, the heat exchanger transferring the heat ofcondensation to a fluid medium and heat from the stream of heatedcoolant to the fluid medium, the condenser further being connected to abuilding by a conduit for transferring the secondary exhaust gas to thebuilding.
 15. The fuel system of claim 14 wherein the secondary exhaustgas is air.